This invention pertains generally to a method for forming patterns on a semiconductor wafer and to an apparatus to practice the method and, more particularly, to maskless projection lithography.
At the present time, the patterns on semiconductor wafers, which represent electronic components and their interconnections, are produced by "writing" the pattern from a mask onto a photoresist covered wafer by a process such as projection lithography. The conventional projection lithographic process employed for producing electronic circuits on semiconductor wafers is similar to exposing a film negative onto photographic paper except that the transferred image is reduced (typically by from 4.times. to 10.times.) by a camera rather than enlarged, thereby making the circuit elements smaller. This process is illustrated in FIG. 1. Light from a light source 110 shines onto a mask 120 containing a circuit pattern. During the process, mask 120 is caused to move in one direction. The light that penetrates the mask pattern, representing the circuit pattern desired to be reproduced, is focused onto a photoresist coated wafer 130 by camera 140 which forms a focused image of the mask demagnified (reduced) by a factor of typically between 4.times. and 10.times.. The wafer 130 is moving in a direction opposite to that of mask 120 such that the image of the mask features are stationary on the wafer.
While effective, this process has numerous drawbacks associated principally with the masks such as the cost of fabricating masks, the time required to produce the sets of masks needed to fabricate microchips, diffraction effects resulting from light from light source 110 being diffracted from opaque portions of the mask, registration errors during mask alignment for multilevel patterns, color centers formed in the mask substrate, defects in the mask or the presence of dust particles on the mask that are rendered as imperfections in the circuit pattern, the necessity for periodic mask cleaning and the deterioration of the mask that follows therefrom. These drawbacks are particularly pronounced during the process of producing prototype microchips. Any minor error or flaw in the circuit design layout or any change in the circuit design can require the fabrication of a full set of masks; an expensive and time consuming process. The aforementioned drawbacks are particularly noticeable when producing small lots of microchips or specialty microchips. What is needed is a method for eliminating the use of a mask in fabricating circuit patterns on semiconductor wafers.
Responsive to these needs the present invention provides a method for producing circuit patterns on semiconductor wafers without the need for a mask.